* UPDATE *
I have placed this function before the prev code
// define the wp_mail_failed callback
function action_wp_mail_failed($wp_error) {
return error_log(print_r($wp_error, true));
}
// add the action
add_action('wp_mail_failed', 'action_wp_mail_failed', 10, 1);
Which helped me track the problem (using headers wrong).
I share this in the hopes this helpes anyone while debugging wp_mail()